The present invention relates in general to radiotelephone communication systems, and more particularly to a method for a radiotelephone to select service from adjacent cells.
The Global System for Mobile Communications (GSM) system requires mobile radiotelephones to monitor adjacent frequencies for the possibility of reselection in case of dropped service. The intent is that the MS decode and synchronize to up to six adjacent carriers, so that the MS has these from which to choose at the time it determines that a reselection to a different carrier is necessary. In recent years there have been a number of new proposals put forth in the GSM standards describing future network features that will allow a network to configure specific carriers so that certain mobile radiotelephone types are prohibited from obtaining service on that carrier. The general direction of these proposals is that specific broadcast data corresponding to a new feature on that carrier allows only new mobile radiotelephone types with the feature enabled to obtain service there. One example of such a feature is a Local Service Area (LSA) which requires a mobile radiotelephone to have a subscription to a specific service area in order to obtain service there. Older mobile radiotelephone types will see the broadcast data as corresponding to an unsuitable carrier, and will not obtain service there. Other suitability criteria include a predefined minimum signal level, accessibility, and a predefined set of specific broadcast information parameters.
However, there is no provision in the GSM specifications for a mobile radiotelephone to select the six adjacent carriers based on the broadcast data of those carriers. The only mechanism in the specifications by which the mobile radiotelephone chooses the adjacent carriers is the relative signal level on the adjacent carriers. For this reason, the list of six adjacent carriers is often referred to as the xe2x80x9csix strongest adjacent cellxe2x80x9d list, or simply xe2x80x9csix strongestxe2x80x9d. No exceptions are provided to include other carriers on which there is the possibility of obtaining service. As a result, when a mobile station is monitoring the six adjacent carriers, it is possible that in future networks, the mobile radiotelephone will consider some or all of these carriers unsuitable for obtaining service. Thus, when the mobile radiotelephone is required to reselect to one of those carriers, there are limited possible carriers from which to choose. In some circumstances this requires the mobile radiotelephone to take more time and resources to reselect to one of those carriers, thus degrading service quality. Further, monitoring adjacent carriers on which there is no possibility of reselection is a waste of mobile radiotelephone resources.
The current GSM specification (GSM 05.08, xe2x80x9cDigital cellular telecommunications system (Phase 2+); Radio Subsystem Link Controlxe2x80x9d, (European Telecommunications Standards Institute (ETSI), European Standard (Telecommunications series), v6.7.1, section 6.6.1.) states the following as required idle mode activities: xe2x80x9cThe list of the six strongest non-serving carriers shall be updated . . . xe2x80x9d, xe2x80x9cThe MS [mobile station] shall attempt to decode the broadcast control channel (BCCH) data block that contains the parameters affecting cell reselection for each of the six strongest non-serving cell BCCH carriers at least every five minutes.xe2x80x9d, and xe2x80x9cThe MS [mobile station] shall attempt to check the BSIC for each of the six strongest non-serving cell BCCH carriers at least every thirty seconds.xe2x80x9d The current GSM specification also requires the mobile station to use these six strongest carriers as candidates for cell reselection, and require that the carrier chosen for reselection is suitable (GSM 03.22, xe2x80x9cDigital cellular telecommunications system (Phase 2+); Functions related to mobile station (MS) in idle mode and group receive modexe2x80x9d, European Telecommunications Standards Institute (ETSI), European Standard (Telecommunications series), v6.2.0, section 5.2.2).
There are no provisions for the mobile radiotelephone to eliminate a cell from its list of six strongest adjacent cells based on suitability criteria. In fact, the current specifications forbid it. The problem scenario becomes more critical when considering future networks operating with mobile radiotelephones that are being built and sold today. FIG. 1 illustrates the current mobile radiotelephone behavior in a prior art network of a GSM system, for example. Given an existing system where there is uniform access, all the carrier cells 10-80 provide the same level of service and a mobile radiotelephone is able to utilize those existing services. In this case, all the carriers 10-80 have all the other carriers of the system in their adjacent cell list. A mobile at position x considers carriers 10, 80, 60, 20, 50 and 40 as its six strongest signals. These are monitored regularly for BCCH and base station identity code BSIC data. Leaving carrier cell 70 moving towards position y, the mobile searches and finds carrier 20 suitable (or 10 or 80), for example, and reselects to it. While entering new cells the mobile radiotelephone re-monitors signal strengths of adjacent cells. As a result, the mobile adds carrier 30 to its list and drops the weakest carrier 60. Later, after moving near to position y, the mobile reselects to carrier 30.
In a future network, it can be assumed that a mobile radiotelephone will require different service access. For example, some networks will not provide data services or some local networks will only have private access for subscribers. In the latter case, referring to FIG. 1, it is assumed that cells 20, 40 and 60 do not provide service access to an older mobile unit (i.e., cells 10, 30, 50, 70 and 80 are available). As before, all the carriers 10-80 have all the other carriers in the system in their adjacent cell list. A mobile, starting at position x, again considers carriers 10, 80, 60, 20, 50 and 40 as its six strongest signals. These are again monitored regularly for broadcast control channel (BCCH) and base station identity code (BSIC), but not the availability of service since this is not required in the standard. In this case, carriers 60, 20 and 40 are unsuitable for reselection as they are not available for service. Leaving carrier cell 70 moving towards location y, the mobile desires to reselect to cell 20 and monitors cell 20 for availability of service. At that time, the mobile determines that carrier 20 is unsuitable and reselect to carrier 80 (or 10) after successfully monitoring cell 80 (or 10) for availability of service. After entering a new cell and re-monitoring signal strengths of adjacent cells, the mobile adds carrier 30 to its list of six strongest signals and drops carrier 60. Later, when leaving carrier 80 and moving near to position y, the mobile monitors carrier 30 for data services and reselects to carrier 30 upon finding those services available.
There are several problems with the mobile behavior in the future network as described above. First, the mobile is wasting idle mode resources monitoring three carriers (20, 40 and 60) which are sufficient in signal strength to be considered as part of the xe2x80x9csix strongest adjacent carriers listxe2x80x9d, but are in fact not suitable candidates for reselection. Secondly, at the time the mobile determines a reselection is necessary (such as when changing cells), there is an additional waste of resources to determine which of the xe2x80x9csix strongestxe2x80x9d is a suitable carrier on which to obtain service. Finally, note that the destination of the mobile is carrier 30, but the mobile could not initially reselect to carrier 30 because it was not in its list of six strongest signals. The xe2x80x9csix strongestxe2x80x9d list actually contained three carriers on which service could not be obtained. Therefore, the mobile had to choose the next best carrier, and then a short time later when it was able to drop a carrier from its list and decode carrier 30, it finally reselected to carrier 30. As a result, the mobile wasted decoding time, monitored carriers it could not use, and made decisions at inappropriate times, i.e., times where the mobile is already busy such as when changing cells.
In view of the above, the need exists for a method to allow a mobile unit to provide a list of not only the six strongest adjacent carriers, but also to augment or modify this list to define suitable adjacent carrier candidates that are actually available for reselection. It would also be of benefit to predefine this listing such that the mobile unit need not search for a suitable carrier at the moment while it is changing cells. Additionally, it would prove beneficial to provide this performance improvement with relatively simple hardware and software at little or no additional cost.